I use the 3.5 cable for the av to my preamp. I cant get any sound when I push the cable pin (3.5) all the way in (if the pin is push all the way in it causes alot of static sound). Its ok if its inserted in half-way, because it is half way in sometimes when ever I accidently move the set the pin get lossen and the sound will goes off.
Are you using the supplied AV Western Digital cable? Any other cable is likely to wired differently which would cause your problem.